remove file cache mechanism

This is my stab at removing the file cache mechanism (see discussion in #130).

- [ ] Documentation needs to be fixed too
- [ ] I'm not sure about the two tests in `test_create.py`. I avoided removing test code I don't fully understand, as it might end up not testing anything :)

It passes most of the tests, failing only the tests in `test_create.py` I haven't updated yet.
This by no means finished, but I am putting it out there to see what happens and get feedback.